Speaker
Description
Ultrarelativistic heavy ions generate intense electromagnetic fields that act as a prolific source of high-energy photons, inducing photonuclear reactions. While these processes are typically studied in ultraperipheral collisions (UPCs), where hadronic interactions are suppressed, an unexpected excess of J/$\psi$ yield at very low transverse momentum ($p_{\rm T}$) has been observed in peripheral Pb--Pb collisions. This signal is attributed to coherent J/$\psi$ photoproduction occurring within the nucleus-nucleus (AA) overlap region. This process serves as a unique probe to investigate the interaction between photoproduced states and the fast-expanding quark-gluon plasma (QGP), potentially unveiling new properties of the medium while simultaneously providing a clean experimental signature to constrain the nuclear gluon distribution at low Bjorken-x. In order to confirm the photoproduction origin of this low-$p_{\rm T}$ excess, polarization measurements are essential. Indeed, the photoproduced quarkonium is expected to retain the transverse polarization of the incoming photon, providing a clear experimental signature to distinguish it from hadronic production. In this contribution, we present centrality-dependent measurements of coherent J/$\psi$ photoproduction in peripheral Pb--Pb collisions at both mid- and forward rapidity and report the first measurement of J/$\psi$ polarization at forward rapidity.
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